Graphene is an atomically thick, two dimensional sheet composed of sp2 carbons in a honeycomb structure. It can be viewed as the building block for all the other graphitic carbon allotropes. Graphite (3-D) is made by stacking several layers on top of each other, with an interlayer spacing of ˜3.4 Å and carbon nanotubes (1-D) are a graphene tube.
Single-layer graphene is one of the strongest materials ever measured, with a tensile strength of ˜130 GPa and possesses a modulus of ˜1 TPa. Graphene's theoretical surface area is ˜2630 m2/g and the layers are gas impermeable. It has very high thermal (5000 W/mK) and electrical conductivities (up to 6000 S/cm).
There many potential applications for graphene, including but not limited to:
(a) additive for mechanical, electrical, thermal, barrier and fire resistant properties of a polymer;
(b) surface area component of an electrode for applications such as fuel cells, super-capacitors and lithium ion batteries;
(c) conductive, transparent coating for the replacement of indium tin oxide; and
(d) components in electronics.
Graphene was first reported in 2004, following its isolation by Professor Geim's group. Graphene research since then has increased rapidly. Much of the “graphene” literature is not on true monolayer graphene but rather two closely related structures:
(i) “few layer graphene”, which is typically 2 to 10 graphene layers thick. The unique properties of graphene are lost as more layers are added to the monolayer and at 10 layers the material becomes effectively bulk graphite; and
(ii) Graphene oxide (GO), which is a graphene layer which has been heavily oxidised in the exfoliation process used to make it and has typically 30 at % oxygen content. This material has inferior mechanical properties, poor electrical conductivity and is hydrophilic (hence a poor water barrier).
There are a variety of methods to produce graphene [Ruoff 2009]. Novoselov et al. produced their first flakes by the mechanical exfoliation of graphite by using an adhesive tape to isolate individual layers [Novoselov 2004]. It has been shown subsequently that graphite can also be exfoliation by using ultrasonic energy to separate the layers when in an appropriate solvent, such as NMP (N-methylpyrrolidone) [Coleman 2008 & 2009].
Wang et al. have shown that ionic liquid are also appropriate solvents for ultrasonic exfoliation. In this case, they mixed graphite powder with ionic liquids such as 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][Tf2N]) and then subjected the mixture to tip ultrasonication for a total of 60 minutes using 5-10 minute cycles. The resultant mixture was then centrifuged [Wang 2010]. Ionic liquids are used to stabilise the graphene produced by the ultrasonication.
Intercalation compounds can be produced by introducing a metal through the vapour phase and then reacting these ions—the layers of the intercalation compound can then be separated by stirring in an appropriate solvent, such as NMP [Valles 2008]. An intercalation approach has also been taken to separate graphene oxide aggregates by electrostatically attracting tetrabutylammonium cations in between the layers of the graphene oxide [Ang 2009]. This technique relies on the charges present in graphene oxide to attract the tetrabutylammonium cations.
Graphene can also be produced by chemical vapour deposition. For example, methane can be passed over copper [Bae 2010]. Alternatively silicon carbide can be decomposed to make a graphene film.
Electrochemical approaches can also be taken to exfoliate the graphene. Liu et al. [Liu 2008] reported the exfoliation of graphite using an ionic liquid-water mixture electrolyte to form “kind of IL-functionalized” graphene nanosheets. Scheme 1 in this paper suggests that the material was produced by the exfoliation of the anode but in their discussion the authors mention the role of the cation. Lu subsequently studied the route in more detail and discussed the possible mechanism involved in the production process [Lu, 2009]. In their paper, they stated “according to the proposed mechanism by Liu, the positively charged imidazolium ion is reduced at the cathode to form the imidazolium free radical which can insert into the bonds of the graphene plane. At the fundamental level, there are several questionable aspects about the radical-insertion mechanism proposed by Liu, especially when the ILs are mixed with water at 1:1 ratio and where an operational voltage as high as 15 V is applied”. Lu et al. showed that the graphene nanosheet production is exclusively at the anode and is due to an interaction of decomposed water species and the anions from the ionic liquid, such as BF4−.
The co-pending international application, published as WO 2011/162727, discloses the formation of graphene using lithium ion exfoliation of graphite, the exfoliation being aided by the insertion of solvent between the layers and sonication. This work is also discussed in a related paper [Wang 2011].
Further methods for the production of graphene are desired—in particular, methods that produce graphene sheets with a controlled number of layers and flake size. Advantageously, the methods should be scalable to allow for the production of graphene on a large scale.